The present invention relates generally to tungsten halogen incandescent lamps having a rounded end type envelope of a high temperature glass and a method of manufacture for said type lamps. More particularly, the present invention relates to improvements made in the high temperature aluminosilicate glass materials employed to form the lamp glass envelope and the exhaust tube portion thereof which have eliminated need to include a refractory metal spiral in said exhaust tube in order to keep the exhaust tube open while the lamp lead-in wires are being hermetically sealed in the lamp glass envelope and which further promotes less residual stress in the sealing region.
A detailed description of the method for manufacture of the above type incandescent lamps is found in issued U.S. Pat. No. 3,798,491, which is assigned to the assignee of the present invention. In said lamp manufacture, a preformed rounded end bulb of high temperature aluminosilicate glass is positioned over mount means which holds an exhaust tube with a refractory metal spiral such as tungsten, lightly embedded in the interior of the tube. Said mount also contains two refractory metal lead wires to which is attached the refractory metal incandescent filament. Heat is applied to the open end of the bulb thereby collapsing the bulb around the lead wires and the exterior portion of the exhaust tube containing the tungsten spiral. The lamp is then flushed with an inert gas to remove impurities and filled with a gas containing a halogen after which the exhaust tube is tipped off by reheating with flames of a gas burner which is generally employed as the heating source.
A number of problems are frequently encountered in this customary method of manufacture. The gas flame reheating employed to tip off the lamp assembly can puncture the exhaust tube leading to failure by entry of the ambient atmosphere into the lamp envelope. The refractory metal spiral employed to maintain the exhaust tube open when the lead-in wires are being hermetically sealed in said envelope by a press seal often extends below the press seal termination which can cause various difficulties during subsequent lamp manufacture and lamp assembly. The press seal formed with said refractory metal spiral is also subject to residual stress which can even produce lamp failure due to glass cracks or air leakage.
An improved high temperature aluminosilicate glass material for the above type incandescent lamps is disclosed in issued U.S. Pat. No. 4,238,705, which is also assigned to the present assignee. Said improved glass composition consists essentially of metal oxides as calculated from the starting batch of materials in the approximate percents by weight 54-71 SiO.sub.2, 12-18 Al.sub.2 O.sub.3, 10-23 BaO, 4-8 CaO, 0-3 R.sub.2 O, wherein R is an alkali metal ion, along with minor amounts of incidental impurities, residual fluxes and refining agents such that the aggregate SiO.sub.2 and Al.sub.2 O.sub.3 content lies within the approximate range 73-83 weight percent, the aggregate BaO and CaO content lies within the approximate range 17-30 weight percent, and the weight ratio between BaO and CaO lies within the approximate range 2.3 to 3.5 so as to provide improved reboil resistance. Said glass material further exhibits an average coefficient of linear thermal expansion 0.degree.-300.degree. C. temperature range between about 37-50.times.10.sup.-7 /.degree. C., a softening point in the 1,000.degree.-1,140.degree. C. range while retaining a comparatively low liquidus temperature not exceeding 1,350.degree. C., and a sealing temperature which generally does not exceed about 1,450.degree. C. Improved press seals are obtained with said glass material to include joinder of refractory metal lead-in wires, generally molybdenum, to larger diameter wires of an iron alloy having greater thermal expansion characteristics in the hermetic seal region of the lamp glass envelope.
It would be desirable to reduce the above-mentioned problems in both manufacture and use of this type incandescent lamp construction without requiring major modifications in the materials being used or lamp structure.